Aside from negative-pressure boosters using a common negative-pressure actuator as a boost source, an electric booster using an electric motor as a boost source is well known as a booster incorporated in a brake device, for example, as described in Patent Document 1. The electric booster described in the Patent Document 1 includes an input piston which moves back and forth by brake pedal operation, a booster piston mounted on the input piston to be movable relative to the input piston, a rotation-linear motion conversion mechanism which makes the booster piston move back and forth, and an electric motor which applies torque to the rotation-linear motion conversion mechanism. The input thrust applied from the brake pedal through an input rod to the input piston and the booster thrust applied from the electric motor to the booster piston generate hydraulic brake pressure within the pressure chamber of a master cylinder.
According to the electric booster of the Patent Document 1, the input piston, the distal end portion of which is inserted in the master cylinder, is divided in the axial direction. A master-cylinder side segment of the input piston is biased toward the master cylinder by a spring device, and thus creates a clearance between the master-cylinder side segment and a brake-pedal side segment of the input piston when braking is not applied. At the initial stage of braking, therefore, the master-cylinder side segment which is a shaft member is applied with the hydraulic brake pressure of the master cylinder and reduces the clearance to the brake-pedal side segment against the biasing force of the spring device. During this process, the hydraulic brake pressure of the master cylinder is not transmitted to the brake pedal, which provides so-called jump-in characteristics in which output is increased, regardless of input. The electric booster, as with the negative-pressure booster, obtains the jump-in characteristics at the initial stage of braking, quickly creates a braking force, and provides a good brake feeling.